The overall goal of this project is to develop antibody therapeutics to treat life-threatening flavivirus infections. Our working hypothesis is that antibodies recognizing specific, highly conserved envelope protein epitopes can prevent and treat infection by several different flaviviruses, including all 4 serotypes of dengue virus and West Nile virus (WNV). Passive immunization is an accepted medical practice to protect against infection. We have used a proprietary recombinant WNV envelope protein (WNV-E) to select 2 recombinant human single-chain variable region fragments (scFv), scFv 11 and scFv 71, from a human phage display antibody library. To increase the avidity and serum half-life of these antibody fragments, we fused each scFv to a human Fc domain to produce bivalent antibodies scFv-Fc 11 and scFv-Fc 71. These antibodies protect mice against a lethal dose of WNV administered either pre-or post-infection. The scFv-Fc 11 and scFv-Fc 71 antibodies also neutralize dengue virus. Antibodies scFv-Fc 11and scFv-Fc 71 therefore, recognize epitopes that are conserved across different flavivirus species. We propose that antibodies 11 and 71 are promising for product development as lead broad-spectrum therapeutics against various flavivirus species and subspecies. In pursuit of antibodies with enhanced therapeutic properties, we will first determine the three-dimensional (3D) structure of dengue virus serotype 2 envelope protein (DEN2-E) in complex with either scFv 11 or scFv 71. This will reveal the extent of the conserved antibody epitopes, and the functional basis for neutralization of both dengue and WN viruses. In parallel, we will seek mutants of DEN2-E and WNV-E that escape neutralization by antibodies 11 and 71. Mapping the escape mutations onto our structure will allow us to design antibodies that are less prone to generating resistant viral strains. In Phase II of this project, we will use our structural data from Phase I to design mutant scFv-Fc antibodies with enhanced epitope binding affinities, a broader spectrum of activity, or enhanced pharmacokinetic properties. We will seek mutated antibodies that neutralize all dengue and WN virus serotypes, and protect mice after infection with these viruses. This project will provide clinicians and public health officials with new means to manage natural, accidental and intentional flavivirus outbreaks. [unreadable] Dengue is a mosquito-borne infection that recently emerged as a major international public health concern. The goal of this research is to develop an antibody therapeutic to treat life- threatening dengue virus infections. This project will provide clinicians and public health officials with new means to manage natural, accidental and intentional flavivirus outbreaks. [unreadable] [unreadable] [unreadable]